1. Field of the Invention
This invention relates to a photoelectric type fire detector of a normal type for transmitting a fire signal when it has detected smoke reaching a level at which the start of a fire is determined or an analog type which detects a physical quantity of smoke and transmits a physical quantity signal and an adjustment unit therefor, and more particularly to a photoelectric type fire detector having a sensitivity adjustment function and an adjustment unit therefor.
2. Description of the Related Art
In order to adjust the sensitivity of a photoelectric type fire detector, a sensitivity adjustment method has been employed which uses a light scattering plate. The light scattering plate is manufactured by adding light reflecting substances, such as metal powder, to a transparent synthetic resin plate in such a manner that light is, by the reflecting substances, scattered in a quantity that is the same as the quantity which is realized by smoke of a density of, for example 10%/m.
Another method has been disclosed by the applicant of the present invention (refer to Japanese Patent Publication No. 4-131538), the method having the steps of: using a scattering and translucent plate which has been manufactured by adding, in place of smoke particles, carbon particles in an arbitrary quantity to a translucent plate made of black and opaque plastic resin, such as AS resin, having a light permeability; and inserting the scattering and translucent plate between a smoke detection light emitting device and a light receiving device.
However, the conventional sensitivity adjustment method using the light scattering plate involves a difficulty in manufacturing the light scattering plate that is adjusted in its quantity of scattered light to a value that corresponds to the smoke density of 10%/m.
In the case where the foregoing method is employed, when the light scattering plate is inserted between the light emitting device and the light receiving device, light emitted by the light emitting device is scattered by the light reflecting substances, such as metal powder, included in the light scattering plate, and the scattered light is incident on the light receiving device as a signal light component. On the other hand, light reflected by the internal wall of the black box passes through the light scattering plate, and light passed through the light scattering plate and reached the internal wall of the black box is reflected by the internal wall. Thus, foregoing light beams are respectively received by the light receiving device as noise components. Therefore, the light receiving device receives light serving as the signal component scattered by the light reflecting substances and light serving as the noise components reflected by the internal wall of the black box.
However, although the light scattering plate is the transparent member, the light scattering plate obscures light when light passes through the same. Therefore, the noise light component reflected by the internal wall of the black box is reduced as compared with the quantity at the time of supervising a fire. Such reduction leads to a fact that the sensitivity or an analog output (the physical quantity of smoke) in fire discrimination is disordered by a degree corresponding to the foregoing reduction. It might therefore be considered feasible to perform correction at the time of the sensitivity adjustment such that the degree of the reduction in the received output is added by means of the light scattering plate. However, there arises a problem in that dispersion among black boxes and slight difference in the positions, at which the light emitting device and the light receiving device are attached, result in that accurate correction cannot be performed.
The other conventional method using the scattering and translucent plate to adjust the sensitivity permits the quantity of addition of the carbon particles to be determined arbitrarily. Furthermore, since the sensitivity is adjusted in accordance with the arbitrary smoke density with respect to the quantity of the addition, the scattering and translucent plate can be manufactured easily.
When the scattering and translucent plate is, in the black box of the fire detector, inserted between the light emitting device and the light receiving device, light introduced from the light emitting device into the scattering and translucent plate is scattered by carbon particles approximating smoke particles. The scattered light is incident on the light receiving device as the signal light component. On the other hand, light reflected by the internal wall of the black box at a position near the light emitting device as compared with the scattering and translucent plate is considerably obscured by the black scattering and translucent plate. Furthermore, light directly passes through the scattering and translucent plate is considerably decayed by the black scattering and translucent plate. In addition, the light is further obscured due to the reflection on the internal wall of the black box. Therefore, the light receiving device receives the noise light reflected by the internal wall of the black box in a quantity that can be substantially ignored as compared with the foregoing method. Thus, the light receiving device receives the light scattered by the carbon particles as the signal light component. Namely, the light receiving device receives only light scattered by smoke in a state where no noise light is present. Therefore, an output accurately representing the received light with respect to an arbitrary smoke density can be obtained.
However, the foregoing method involves a fact that the light receiving device receives the noise light component reflected by the internal wall of the black box when the light emitting device emits light at the time of supervises a fire. Thus, the sensitivity or the analog output (the physical quantity of smoke) to discriminate a fire is disordered by a degree corresponding to the received noise light component. As a result, it might therefore be feasible to perform correction by adding the noise light component at the time of adjusting the sensitivity by means of the scattering and translucent plate. Similarly, there arises a problem in that dispersion among black boxes and slight difference in the positions, at which the light emitting device and the light receiving device are attached, result in that accurate correction cannot be performed.